In power station--and industrial heating plants, waste gases are used in a regenerative heat exchanger for preheating the combustion air. In this process, e.g., a nitrogen oxide (NO.sub.4) contained in a waste gas can be further reduced, when the heat-storage material of the regenerative heat exchanger, which may be stationary or circulating, is completely or partially formed of a catalytically acting material, and, e.g., first of all ammonia (NH.sub.3) is added thereto as a reducing agent. Thus, it is a case of a catalytic nitrogen oxide reduction, in which the reduction (deoxidation) of the nitrogen oxide is effected by addition of NH.sub.3 in the presence of a catalyst integrated into a regenerative air heater or a regenerative heat exchanger. As a rule, the NO.sub.x -containing waste gas is a crude gas of a furnace, which passes at an end of a steam generator for preheating the combustion air that flows through the regenerative heat exchanger. It is known to this end, for selective reduction of nitrogen oxides in waste gases from furnace apparatuses, to add NH.sub.3 in a vapor state, in mixture with air as a carrier gas, under pressure, or in water, without pressure, to waste gases exiting a furnace apparatus. Attempts were made to obtain a strand-free ammonia-and temperature distribution in a waste gas stream before its entry into the catalyst by providing a corresponding structure inside of an extending waste gas channel. The catalyst or the heat-storage material is connected in series with a rotating regenerative heat exchanger for transmitting the heat of the waste gas to the combustion air fed to the furnace, with the optimal reaction temperatures inside the waste gas stream being taken into account. Fixed-bed catalysts especially proved themselves for a vertical descending stream of a waste gas, of which several interchangeably were used for acting on the waste gases for removing nitrogen. The fixed-bed catalysts, which were formed as a honeycomb structure, contained as a catalytically acting material vanadium compounds, which fostered the reaction of the nitrogen oxide with NH.sub.3, which was previously supplied into the waste gas stream and mixed therewith in a small proportion on the way of the waste gas to the catalyst. The reaction with the nitrogen oxides, which are contained in the waste gases, results essentially in production of molecular nitrogen and water as reaction products, which can be let off harmlessly into the environment.
European patents 0 195 075 and 0 257 024 disclose that a reducing agent is added to the crude-, flue- or waste gas side, or to the pure gas-or air side, or to the gas side as well as to the air side. The NH.sub.3 is thus admixed either to the flue gas before it enters the catalyst, or to the preheated fresh air before it enters the catalyst, or to the both. In any way, compounds contained in the flue gas, namely, NO.sub.x, are catalitically transformed into harmless components. With flue gas side addition, the NH.sub.3 and the toxic compounds NO.sub.x have a too short dwell time in the catalyst, so that only an incomplete reaction occurs. The non-consumed NH.sub.3, therefore, is transported to the side with a purified flue gas which flows into the chimney, so that an undesirable slippage of non-consumed reducing agent into the environment occurs by emission through the chimney. With the air side addition of the NH.sub.3, as a rule, a leakage occurs and, despite presence of seals, a certain amount of NH.sub.3 is transported from the pure gas side to the purified flue gas side. This amount is therefore lost and burdens the adjacent portions of the apparatus.